Typical optical fiber telecommunication systems and networks utilize fiber optic assemblies such as cables, cable assemblies and network components. Certain portions of such systems and networks utilize multimode optical fibers (“multimode fibers”), while other portions utilize single-mode optical fibers (“single-mode fibers”).
Multimode fibers have larger cores than single mode fibers and thus can support many more guided modes. However, because of modal dispersion and modal noise, multimode fibers are typically used for short-distance applications, such as communicating between devices in data centers, and usually operate at wavelengths of 850 nm and sometimes 1300 nm. Single-mode fibers are typically used for long-distance applications, such as communicating between data centers, and usually operate at wavelengths of 1310 nm or 1550 nm.
To ensure the operability of telecommunication equipment that uses multimode fibers, standards for measuring multimode fiber attenuation have been established by the Telecommunications Industry Association (TIA) and by the International Electrotechnical Commission (IEC). For example, the standards set forth in IEC 61280-4-1:2009(E) are applicable to attenuation measurements of installed multimode fiber optic cabling for lengths of up to about 2,000 m.
Attenuation measurements can be made by calculating the “encircled flux” of the “launch” multimode fiber cable. A launch multimode fiber cable is connected to a light source and is used to launch light into the installed cabling, as described in the publication TIA TSB 178, entitled “Launch condition guidelines for measuring the attenuation of installed multimode cabling,” which document is incorporated by reference herein. The encircled flux is defined as the fraction of cumulative near-field power emitted by the multimode fiber to the total output power of the fiber as a function of radial distance from the optical center of the fiber core. The upper and lower bounds on the encircled flux over a range of fiber radii define an “encircled flux template,” which has a region (area) of acceptable performance.
Unfortunately, it is impractical for field technicians to directly measure the encircled flux from the launch multimode fiber cable because it requires a measurement of the near-field power emitted by the multimode fiber. This measurement is difficult to conduct in the field and the equipment used is expensive and environmentally sensitive. However, encircled flux can be measured indirectly by providing a series of offset connections to launch multimode fiber and measuring the transmitted power for each such offset connection. The offset connections produce differential modal attenuation, which can be compared to values when tested by a launch multimode fiber cable known to be at or very close to the encircled flux target. Consequently, limits on the attenuation are easier to measure and are typically used to field test the performance of installed multimode fibers.
To test a launch multimode fiber cable, the IEC recommends that the field technician carry multiple (e.g., a set of three) different jumper cables (“jumpers”) each with a different number of fiber offsets. The first jumper has no offset and is connected to launch multimode fiber cable, and a first power measurement is made using a power meter. This power measurement ensures the launch connector is of reference quality and serves as a baseline for the attenuation measurement. The first jumper is removed and a second jumper or set of jumpers having two offsets is then connected to the launch multimode fiber cable. A second power measurement made using a power meter. The second jumper cable is removed and a third jumper cable having five offsets is then connected to the launch multimode fiber cable. A third attenuation measurement is made. The power measurements from all of the different jumper cable connections are then compared to established attenuation values (based on the aforementioned encircled flux template) to determine if the multimode launch system (i.e., the launch multimode fiber cable and the light source to which it is attached) meets the IEC and/or TIA standard for attenuation.